4. THE COCCIDEA
Members of the phylum Apicomplexa are all parasites. There are following Characteristics of this phylum:
1. They have apical complex for penetrating host cells.
2. They have single type of nucleus.
3. They have no cilia and flagella, except in certain reproductive stages.
4. The life cycles include asexual stages: Schizogny, Sporogony, Gametogony; Sexual phases.
They produce a resistant spore or oocyst after sexual reproduction. Some members of this class like Plasmodium, Cryptospridium, Toxoplasma, Cyclospora and Babesia causes disease in domestic animals and humans.
Life cycle: Many coccidian are intracellular parasites. Their life cycles have three phases.
(a) Schizogony or merogony: It is multiple fission of an asexual stage in host cells. It forms many merozoites. Merozoites leave the hose cell and infect many other cells.
(b) Gametogony: Some of the merozoites undergo gametogony. It is start of sexual phase of the life cycle. The parasite forms microgametocytes or macrogametoeytes. Microgametocytes undergo multiple fissions. It produces biflagellate microgametes. Microgametes come out from the infected host cell. The macroaametocyte develops directly into a single macrogamete.A microgamete fertilizes a macrogamete to produce a zygote. The zygote enclosed in a capsule. Now it is called oocyst.
(c) Sporogony: The zygote undergoes meiosis. The resulting cells divide repeatedly by mitosis. This process, called sporogony. It produces many sporozoites in the oocyst. Sporozoites are rod like. The new host ingests the sporozoites. Now they infect the cells of a new host. It digests the oocyst. Sporozoites are also introduced by a mosquito bite indirectly.
Disease caused by Coccideans
(a) Malaria: Caused by Plasmodium
Plasmodium causes malaria. It has a long history. It was present back in 1550 A.D. The Crusades failed during the medieval era due to malaria and typhus. These diseases destroyed the armies. Now over 100 million humans are annually infected by malaria.
Life cycle of plasmodium: The Plasmodium has two hosts: vertebrate (human) and mosquito. Schizogony occurs in liver cells and red blood cells. Gametogony also occurs in red blood cells. An ther mosquito sucks blood and takes in gametocytes into its body. The gametocytes fuse and form zygote. The zygote penetrates the gut of the mosquito. It is changed into an ooeyst. Sporogony form haploid sporozoites. These sporozoites are ready to enter a nets host.
The symptoms of malaria recur periodically. These are called paroxysms. The maturation of parasites, rupturing of red blood cells and release of toxic metabolites cause chills and fever.
Fig: Life cycle of plasmodium
Species of Plasmodium: Four species of Plasmodium causes malaria in human.
1. P. vivax: Its paroxysms recur every 48 hours. This species occurs in temperate re ions. It has been nearly eradicated in many parts of the world.
2. P. falciparum: It causes the most virulent form of malaria in humans. Its paroxysms an irregular. It is now mainly present in tropical and subtropical regions. It remains on of the greatest killers humanity in Africa.
3. P. malariae: It is worldwide in distribution. Its paroxysms recur every 72 hours.
4. P. ovale: It is the rarest of the four human malarial species. It is primarily tropical in distribution.
(b) Coccidiosis: Caused by Coccidians
Coccidians cause coccidiosis. It is a disease of poultry, sheep, cattle, and rabbits. Two genera of coccidians, Isospora and Eimeria are important parasites of poultry. They cause huge losses to poultry in U.S.
Cryptosporidium (a coccidian) causes chronic diarrhea in AIDS patient It resists chlorination. It is most virulent in immunosuppressed individuals.
Toxoplasmosis is a disease of mammals (including human) and birds. Sexual reproduction of Toxoplasma occurs in cats feces. Oocvsts are ingested with food contaminated by cat feces. They can also be ingested by eating meat containing encysted merozoites. Most infections in humans are without any symptom. An effecti e immunity develops
after infection. But sometimes. a woman is infected near the time of pregnancy or during pregnancy. Congenital toxoplasmosis may develop in a fetus. Congenital toxoplasmosis is a major cause of still births and spontaneous abortions.
Some fetuses mav survive. But it show signs of mental retardation and epileptic seizures. Congenital toxoplasmosis has no cure. Stray and pet cats should he kept away from children’s sandboxes by using sandbox covers. Awareness should be created in couples wants to having children. They should not have to eat raw mutton or beef.